Fiber-reinforced composite materials having an epoxy resin composition as a matrix resin are widely used for aircrafts, automobiles and industrial applications due to their excellent mechanical properties and the like. In particular, in structural materials and interior materials for aircrafts, from the viewpoint of weight reduction, use of fiber-reinforced composite materials as face sheets of honeycomb panels is increasing. As the matrix resin, a resin composition primarily including an epoxy resin and an amine curing agent is frequently used, and its cured material exhibits high toughness and provides the characteristic of a high heat deformation temperature. However, as applications of composite materials are broadened, needs for the exhibition of higher mechanical characteristics are greatly increasing.
As one measure for the needs, Patent Document 1 proposes that dicyandiamide be added into an epoxy resin composition as a curing agent to thereby improve interlaminar shear strength and flexural strength. However, if dicyandiamide is used, it easily dissolves in an epoxy resin in a temperature raising process, increasing its reactivity with the epoxy resin, and thus tends to promote curing reaction with the epoxy resin due to temperature increase during resin film production or prepreg impregnation, thereby posing the problem of continuously increasing the viscosity of the resin composition. In addition, curing reaction proceeds during the storage of the prepreg in a working environment, causing the problem that tackiness and drapability of the prepreg are likely to decrease.
On the other hand, demanded is a self-adhesion technique for directly bonding a prepreg of fiber-reinforced composite materials to a honeycomb core without use of a film adhesive in order to make the honeycomb panel further light weighted and to reduce molding cost.
However, the self-adhesive strength of a prepreg needs to be improved by a matrix resin in order to eliminate the need of a film adhesive, and, in order to improve the self-adhesive strength, it is important to obtain a fillet having a favorable shape and strength, which is formed in the bonding surface between a honeycomb core and a prepreg. The strength of a fillet depends on the toughness of the cured material of a matrix resin and the shape of a fillet is closely related to the viscosity of a matrix resin in heat curing; the higher the minimum viscosity, the better the shape of a fillet to be obtained. In other words, if the toughness of a cured resin material and the resin viscosity in heat curing are not appropriate, sufficient adhesive strength caused by a fillet is not obtained.
In addition, prior to heat curing of a prepreg, the viscosity of a matrix resin is preferably low. This is because if the resin viscosity is low in a normal temperature region in which a prepreg is treated, tackiness and drapability can be keep good. This is also because, in a step of fabricating a resin film prior to prepreg impregnation, if the resin viscosity is low in the temperature range of roughly from 60 to 90° C., the production efficiency of a prepreg can be improved.
Patent Document 2 describes that, when the prepreg is directly bonded to a honeycomb core, the use of dicyandiamide along with an amine curing agent as a curing agent for an epoxy resin composition to be a matrix resin of a prepreg enables the formation of a good strength fillet, thereby improving adhesion strength. However, when dicyandiamide is used in combination with an amine curing agent in this manner, the reaction activity of dicyandiamide with an epoxy resin is increased as described above, and therefore the dicyandiamide is prone to cause curing reaction with the epoxy resin even by a slight temperature increase. For example, the resin viscosity continuously increases during resin film fabrication, thereby causing the problem of lowering the production efficiency of a prepreg. Additionally, during the storage of a prepreg in a working environment its curing reaction proceeds, causing the problem that tackiness and drapability of the prepreg are likely to decrease. In addition, although this matrix resin improves the toughness of the cured resin material to some extent, the resin is still insufficient to improve toughness for improving the strength of a fillet formed when a honeycomb is directly bonded to a prepreg. For this reason, this resin is insufficient in mechanical properties.    Patent Document 1: Japanese patent application Kokai publication No. Hei 2-51538    Patent Document 2: Japanese patent application Kokai publication No. Sho 58-83022